1. Field of the Invention
This invention relates generally to amplifiers, and more particularly is directed to improvements in amplifiers having change-over switches associated therewith for selecting different operating modes.
2. Description of the Prior Art
Various types of apparatus exist in which an amplifier is associated with circuits having change-over switches driven by relays for selecting the operating mode of the apparatus. For example, in existing tape recording and reproducing apparatus, relays are energized for selecting the recording mode of operation so that, if the apparatus is inadvertently or erroneously disconnected from the usual AC power source during a recording operation, the relays are deenergized to return the change-over switches to their positions for the reproducing or play-back mode of operation. Further, the existing apparatus usually has a first smoothing power circuit for supplying DC power to operate those elements, such as, the amplifying circuit, in which relatively small currents flow, and a second smoothing power circuit for supplying DC power to energize those elements, such as, the change-over switch relays, bias oscillator and end-of-tape alarm, in which relatively large currents flow, with the first and second smoothing power circuits having relatively large and small discharging time constants, respectively, when disconnected from the AC power source. As a result of the foregoing, if the apparatus is inadvertently disconnected from the AC power source while in the recording mode, the change-over switches are returned to the positions for the reproducing mode while the DC power applied to the amplifying circuit is still sufficient to cause operation of the latter, with the result that "howling" occurs. If such howling is to be avoided, it is necessary that the change-over from the recording mode to the reproducing mode be delayed, following inadvertent disconnection from the AC power source, until the DC voltage applied to the amplifying circuit, and particularly to the preamplifier therein, has declined to a level at which at least the pre-amplifier is no longer operative.
Similarly, in other types of apparatus including an amplifier associated with circuits having relay driven change-over switches for selecting the operating mode, it is desirable that the responses of such change-over switches to inadvertent disconnection from an AC power source be delayed or occur in a predetermined order.